Good morning. The only current offering of stackable switches is the Catalyst 3750. No other switch supports stacking.
I have heard that there is going to be an upgrade of the 3750 from a 32Gbps backplane to a 64Gbps backplane. You will be able to mix a 32Gbps 3750 with a 64Gbps 3750, but you will only get 32Gbps on the backplane.